How do I determine Shift+Left Click modal operator?
I may be wrong.. but this is my understanding (and it seems to work for me):

There is nothing as simultaneous with the events. Even if you press shift and click simultaneously, there are actually four separate events. 1) event.type = 'LEFT_SHIFT' with event.value = 'PRESS' 2) event.type = 'LEFTMOUSE' and event.value = 'PRESS' 3) event.type = 'LEFT_SHIFT' &amp; event.value = 'RELEASE' 2) event.type = 'LEFTMOUSE' and event.value = 'RELEASE' (the order might change depending on what comes first). 

There is also a CLICK event.value available for LEFTMOUSE event.type that combines PRESS and RELEASE.

So, to answer your question, you would basically need to store an instance variable like self.shift and set / reset its value on event.value == 'PRESS' / event.value == 'RELEASE'. You can check in your function whether with the mouse press event occurred when self.shift was True.

If you figure out a better way to do this, I would be interested to know :)